Crack Initiation Conditions Research Articles

Research on high‐pressure abrasive water jet slotting and pressure relief technology for hard rock roof

AbstractTo solve the problem of severe rock pressure near coal mining face tunnels, a high‐pressure abrasive water jet slotting and roof breaking pressure relief technology is proposed. First, the laneway deformation mechanism and the process of hard rock slotting using high‐pressure abrasive water jets under long‐distance cantilever conditions are analyzed, and the crack initiation conditions of roof strata are obtained. Second, slotting tests under different slotting pressures, nozzle diameters, abrasive particle sizes and slotting times were carried out, and the slotting parameters of a high‐pressure abrasive water jet on a typical roof rock were obtained. Finally, industrial application was carried out in the 81,403 working face of Huayang No.1 Mine. After the hydraulic roof slotting measures were implemented in the test area, the maximum axial force of the anchor cable was reduced to 67 kN, which was 35.5% lower than that of the comparison. The average stress of the coal seam was 15 MPa, which was approximately 25% lower than that of the comparison. The deformation of the tunnel in the experimental area was significantly controlled, with an average movement of 30.0% toward the roof and floor of the tunnel and an average movement of 23.2% toward the two sides of the tunnel. Compared with the movement in the comparison section, the movement toward the roof and floor of the laneway was 42.3% lower, and the movement toward the two sides was 38.2% lower. The industrial application results show that high‐pressure abrasive water jet roof slotting and pressure relief technology can cut off the stress transmission path between the roof rock on both sides, effectively improve the stress state of the surrounding rock of the laneway, reduce the deformation of the roof, floor and two sides of the working face in the later stage of the mining laneway.

Pipeline Circumferential Cracking in Near-Neutral pH Environment Under the Influence of Residual Stress: Dormancy and Crack Initiation

The purpose of this study is to identify the integrity challenges encountered by buried pipeline steels, specifically to address Circumferential Near-Neutral pH Corrosion Fatigue (C-NNpH-CF). Damage to the pipeline’s protective coating and corrosion conditions increase the risk of service failures caused by C-NNpH-CF. (Note that this mechanism has previously been termed near-neutral pH stress corrosion cracking.) Unlike axial cracking, circumferential cracking is primarily influenced by residual stress from pipeline bending, geohazards, and girth welds. External corrosion pits often lead to dormant cracks, with growth ceasing around 1 mm depth due to reduced dissolution rates. Investigating the impact of bending residual stress (an appropriate source of axial residual stress) and cyclic loading (simulated pipeline pressure fluctuation), the study employs the digital image correlation (DIC) method for stress distribution analysis. Factors like applied loading, initial notch depth, and bending conditions influence crack initiation and recovery from the dormancy stage by affecting stress distribution, stress cells, and stress concentration. Cross-sectional and fractographic images reveal time/stress-dependent mechanisms governing crack initiation, including dissolution rate and hydrogen-enhanced corrosion fatigue. The study emphasizes the role of various residual stress types and their interactions with axial cyclic loading in determining the threshold conditions for crack initiation.

New insights into the effects of low temperatures on the stress-optical properties of printable photopolymers: An experimental investigation

The application of 3D printing technology for replicating natural rock models and achieving the visualization and quantification of hidden structural changes and stress evolution in underground rocks holds significant importance for engineering disaster prevention as well as the development of oil and gas resources. The key to achieving this lies in developing model materials suitable for 3D printing that accurately depict the physical and mechanical properties of rocks. In this paper, the impact of temperature cycle treatment on the mechanical properties of printing material VeroClear was tested and the effect of temperature on the stress-optical properties of VeroClear was analyzed. By using 3D printing models with pre-existing cracks, the crack initiation conditions, propagation laws and stress field distribution characteristics of stress-sensitive materials at different temperatures were obtained. The results show that the mechanical properties of the freshly printed VeroClear model exhibit instability due to incomplete curing. Heat treatment can facilitate complete curing and enhance their mechanical characteristics, whereas cold treatment fails to yield similar improvements. Lower ambient temperatures reduce photoelastic fringe susceptibility to stress, yet allow accurate quantification of full-field stress within models by VeroClear. Lowering the temperature significantly enhances the brittleness of VeroClear, enabling printed models to demonstrate crack evolution characteristics similar to those observed in actual rocks. By analyzing the evolution of photoelastic fringe, it is found that the wing crack propagates along the direction of the local minimum principal stress difference near the crack tip. These findings provide a foundation for utilizing stress-sensitive materials with enhanced brittleness in low-temperature environments for simulating rocks and quantitatively analyzing their fracture behavior and the role of stress field control.

Effect of Carbides on Thermos-Plastic and Crack Initiation and Expansion of High-Carbon Chromium-Bearing Steel Castings

The bearing steel’s high-temperature brittle zone (1250 °C–1100 °C), second brittle zone (1100 °C–950 °C), and low-temperature brittle zone (800 °C–600 °C) were determined by the reduction in area and true fracture toughness. The crack sensitivity was strongest at temperatures of 1200 °C, 1000 °C, and 600 °C, respectively. Various experimental and computational methods were used to establish the phase type, microstructure, size, and mechanical properties of carbides in bearing steel. The critical conditions for crack initiation in the matrix (FCC-Fe, FCC-Fe, and BCC-Fe)/carbides (striped Fe0.875Cr0.125C, netted Fe2.36Cr0.64C, and spherical Fe5.25Cr1.75C3) were also investigated. The values for the high-temperature brittle zone, the second brittle zone, and the low-temperature brittle zone were 13.85 MPa and 8.21 × 10−3, 4.64 MPa and 6.52 × 10−3, and 17.86 MPa and 1.86 × 10−2, respectively. These were calculated using Eshelby’s theory and ABAQUS 2021 version software. The ability of the three carbides to cause crack propagation was measured quantitatively by energy diffusion: M3C > MC > M7C3. This study analyzed the mechanism of carbide precipitation on the formation of high-temperature cracks in bearing steel casting. It also provided the critical conditions for carbide/matrix interface cracks in bearing steel continuous casting, thus providing effective support for improving the quality of bearing steel casting.

Influence of accelerated corrosion on Al/steel RSW joints by in situ compression tests

The present work investigates the correlations between galvanic corrosion, intermetallic compound (IMC) formation, and the input of welding energy with respect to the initiation and propagation of micro-cracks in micropillars of resistant spot welding (RSW) joints between aluminum (Al) and steel. The present results indicate that, in the high welding energy region, Al was corroded first after 26 cycles of corrosion, and Al5Fe2 IMC was corroded after 72 cycles of corrosion due to its high corrosion potential predicted by the calculations based on density functional theory (DFT) in terms of the Nernst equation. In comparison with the high welding energy region, less corrosion was observed in the middle welding energy region due to the thinner Al5Fe2 IMC layer, a lower amount of Al13Fe4 IMC in the Al matrix, and lower residual stress. Mechanical properties at different locations after various corrosion conditions were obtained using in situ compression tests, including the stress-strain responses and the strain rate sensitivity. The micropillars from the high welding energy region have a higher average yielding stress due to the thicker IMC layer than those from the middle welding energy region. The yielding stress decreases gradually with increasing corrosion cycles. Three conditions for crack initiation and propagation have been identified: firstly, for pillars from the high welding energy region before corrosion or the middle welding energy region (before or under the minimal corrosion conditions), the cracks initiate within the IMC layer; secondly, for pillars from the high welding energy region after 26 cycles of corrosion (under the moderate corrosion condition), cracks propagate at the Al/IMC interface; finally, for pillars from the high welding energy region after 72 cycles of corrosion (under the severe corrosion condition), the cracks initiate at the IMC/steel interface.

Influence of adhesive rheological behavior on stable mode II crack propagation in bonded interface subjected to shear loading condition

Bonded joints exhibit high mechanical performances but also suffer from poor durability in humid and/or hot environmental conditions. Indeed, delayed failures can be observed under stationary loading conditions due to chemical aggression but also due to creep failure. Despite bonded joints are generally loaded in shear, discussion on crack initiation and propagation condition under mode II stationary loading condition are lacking. Then, a semi-analytical model is proposed here to determine response of a End loaded Split (ELS) like test specimen considering viscoelastic-viscoplastic interface behavior, stationary loading condition and steady-state crack propagation regime. From these simulations, the evolution of crack propagation rate as a function of macroscopic loading intensity is obtained. The intrinsic nature of such “master curves” is finally discussed.

Study on root strain concentration of girth weld joint with variable wall thickness for X80 pipe

In this study, considering the particularity of variable wall thickness joints, a strain concentration characterization method for variable wall thickness joint is proposed. The finite element simulation method is used to establish a finite element model consistent with the physical joint, and calculate the root strain concentration under different misalignment, different weld strength matching, different pipes strength collocation and different heat affected zone (HAZ) softening conditions. Based on the aforementioned analysis, it is found that the misalignment, the weld strength matching, the pipes strength configuration and HAZ softening all affect the strain concentration at the root of the joint. In particular, the misalignment is easy to cause geometric structure mutation of root, which is the main factor leading to strain concentration for root of weld. The larger misalignment, the lower strength matching of the weld, and the higher pipes strength can increase the strain concentration at the root. However, a large root strain concentration can induce strain aging embrittlement or coupling with welding defects to cause material damage, providing conditions for root crack initiation and propagation.

CRACK INITIATION AT GRAIN BOUNDARY WITH A VARIABLE MISORIENTATION VECTOR

A model is proposed for the initiation of cracks at “knife” grain boundaries, i.e., rectilinear, broken boundaries of deformation origin, oriented along the tension axis, arising as a result of rotational incompatibility in the form of a disclination dump at the stage of pre-fracture of the material. The disorientation of the “knife” boundary decreases down to the breaking point in the body of the grain, and its decrease (rotational instability damping) occurs not continuously, but discretely through the branching of secondary low- and high-angle boundaries. In this case, along the “knife” boundary, at the points of its branching, linear mesodefects of the rotational type arise-wedge disclinations of the same sign. A model of crack initiation in an elastic field of a chain of negative disclinations that create tensile elastic stresses is proposed. When analyzing the conditions for crack initiation, a combined criterion was used, according to which the simultaneous fulfillment of force and energy conditions was required for crack initiation. Within the framework of this model, the critical values of the total value of the Frank vector of disclinations located on the “knife” boundary and the average gradient of its misorientation are calculated, upon reaching which the criterion of crack initiation is satisfied. The dependences of these quantities on the length of the “knife” boundary and the effective screening radius of the elastic field of disclinations are calculated. It is shown that the appearance of cracks is most likely at the boundaries with the largest misorientation gradient (exceeding 8–20 deg/?m, depending on the length of the boundary).

The micromechanics‐based rate‐dependent constitutive model of flawed rocks at intermediate strain rate

AbstractIn the previous works conducted by Zhou and Yang, the deformation induced by wing crack coalescence was not investigated. In this paper, a novel micromechanics‐based rate‐dependent constitutive model is proposed to investigate the effect of wing crack coalescence on the mechanical properties of rocks at intermediate strain rate. The rate‐dependent damage mechanism is revealed using the frictional sliding crack model. Wing crack interaction is defined using pseudo‐traction method as well as the dynamic stress intensity factors. Strain energy density factor approach is applied to determine the critical condition of wing crack initiation, propagation, and coalescence. The strain softening behaviors of flawed rocks is studied by using thermodynamic theory.

Effect of macrozones on fatigue crack initiation and propagation mechanisms in a forged ti-6Al-4V alloy under fully-reversed condition

In the present study, the mechanisms of fatigue crack initiation and propagation was investigated in a forged Ti-6Al-4V alloy with a bimodal microstructure through a combination of experiments and crystal plasticity finite element simulations. Transgranular crack initiations were observed on both basal and prismatic planes, in grains exhibiting high plastic activity. A fatigue indicator parameter based on the Tanaka-Mura model was found to correctly identify these critical grains. Intergranular crack initiations were found in pairs of grains sharing the same basal plane. An analysis of the grain boundary configuration suggested that these grains were separated by a (0001) tilt subgrain boundary as the c-axis was argued to belong to the boundary plane. Short crack propagation was found to be transgranular and crystallographic on either basal or prismatic plane depending on the microtextured regions they belong. Crack path was more serrated with high number of deflection and branching in regions favoring prismatic activity. However, no significant difference in terms of crack propagation rate was noticed between the different macrozones which was attributed to a relatively similar plastic activity in both regions. An analysis of the M-index finally suggested that the presence of a macrozone was not a necessary condition for crack initiation but appeared necessary for its propagation.

Simulation of dynamic fracture in quasi-brittle materials using a finite element modeling approach enhanced by moving mesh technique and interaction integral method

This work presents an efficient modeling approach for predicting dynamic crack propagation mechanisms in quasi-brittle materials. The proposed method consists of a standard Finite Element code enhanced by Moving Mesh (MM) technique consistent with the Arbitrary Lagrangian-Eulerian (ALE) formulation. The MM is used to reproduce the geometry evolution caused by dynamically growing cracks. More precisely, the nodes of the computational mesh around the crack tip change position during the simulation consistently with the growth of the crack front. In such a context, the ALE formulation ensures an effective re-location of mesh nodes inside the computational domain, reducing the overall amount of remeshing events. The motion of the computational mesh takes place according to previsions dictated by classic fracture criteria, which define crack initiation conditions, the direction of propagation, and the velocity of the advancing crack front. Such conditions depend on Dynamic Stress Intensity Factors at the crack front. Hence, accurately predicting these fracture variables is mandatory to ensure consistent mesh motions. To this end, the proposed approach implements the ALE formulation of the M-integral, which enables computing fracture variables on moving elements without losing accuracy. The validity of the proposed modeling strategy has been assessed through comparisons with numerical data and analytical formulations reported in the literature.

EBSD Characterization of the Microstructure of 7A52 Aluminum Alloy Joints Welded by Friction Stir Welding.

The microstructure and texture of materials significantly influence the mechanical properties and fracture behavior; the effect of microstructure in different zones of friction stir-welded joints of 7A52 aluminum alloy on fracture behavior was investigated in this paper. The microstructural characteristics of sections of the welded joints were tested using the electron backscattered diffraction (EBSD) technique. The results indicate that the fracture is located at the advancing side of the thermomechanically affected zone (AS-TMAZ) and the stir zone (SZ) interface. The AS-TMAZ microstructure is vastly different from the microstructure and texture of other areas. The grain orientation is disordered, and the grain shape is seriously deformed under the action of stirring force. The grain size grows unevenly under the input of friction heat, resulting in a large amount of recrystallization, and there is a significant difference in the Taylor factor between adjacent grains and the AS-TMAZ–SZ interface. On the contrary, there are fine and uniform equiaxed grains in the nugget zone, the microstructure is uniform, and the Taylor factor is small at adjacent grains. Therefore, the uneven transition of microstructure and texture in the AS-TMAZ and the SZ provide conditions for crack initiation, which become the weak point of mechanical properties.

On the fretting fatigue crack nucleation of complete, almost complete and incomplete contacts using an asymptotic method

In this work, the fretting crack nucleation of an aeronautical Ti-10V-2Fe-3Al titanium alloy is investigated. The main difficulty when designing complex industrial systems, especially helicopters rotor parts, lies in the prediction of the assembly interfaces behaviour. Those areas can be subjected to fretting loading characterized by a very confined multiaxial high stress gradient, leading to fatigue crack nucleation. This kind of failure reduces the parts potential lifetime and increases therefore the security margin that can be considered.The aim of this paper is to provide a criterion to prevent crack nucleation on industrial contacts configurations. To reach this objective, this study focuses on the geometrical conditions, loadings and material properties linked to crack initiation. Two dedicated fretting test campaigns are used to study crack initiation conditions with various contact shapes and loadings on the same material. The crack initiation conditions are compared with a contact mechanics representation based on asymptotic methods. This method allows the stress field evaluation at the hot spot location by using the problem’s eigenvalues.The combination of the asymptotic method with contact mechanics and the Goodman approach allows to represent the key parameters of the fretting fatigue crack initiation. They include the impacted material volume, the shear intensity and the bulk stress. A fretting fatigue crack initiation criterion is proposed. It is based on classical material properties and the bulk stress ratio Rσ0 only and is applicable to all sorts of contact shapes. The main interests of this prediction method are the absence of size effect parameters for industrial contacts and its efficiency, as it is only based on analytical formulations.

On the intermetallic constituents in the sodium-induced edge cracking of hot-rolled AA5182 aluminum alloys

ABSTRACT The productivity and recovery of hot-rolled Al-Mg-Mn sheets often suffer from sodium-induced edge cracking. In this study, AA5182 ingots with different levels of Na were prepared by direct-chill (DC) casting, and were then subsequently subjected to homogenisation and hot-rolling processes. The edge crack occurrence during hot rolling in the pass #11 (84% reduction) and pass #18 (96% reduction) was investigated, with special attention paid to the impact of intermetallic constituents in the ingot subsurface region, where all edge cracks were initiated. Correlations between the susceptibility to edge cracking, enriched intermetallic constituents in the inverse segregation zone, Mg2Si cavitation, meniscus bands, and Na contamination were observed. The results revealed that Fe-rich intermetallic particles were not involved in crack formation, while Mg2Si particles had a strong tendency to debond from the Al matrix and form cavities at the edges of the rolled strip. The higher the Na content, the more severe the Mg2Si cavitation, and consequently, the higher the susceptibility to edge cracking. Numerous debonded Mg2Si particles contaminated with Na in the subsurface region provided favourable conditions for crack initiation and propagation. The role of the meniscus bands on the large and profound edge cracks was also elaborated.

3D assessment of surface influence on crack initiation in sharp notches under a mixed mode of loading

Sharp notches occur in constructions and lead to stress concentrations. Thus they are dangerous points of constructions and must be properly evaluated. In the present study the effect of a free surface on crack initiation conditions is observed and evaluated by means of a 3D model of a specimen with a sharp notch and by means of the finite fracture mechanics of general singular stress concentrators (GSSCs). The effect of a free surface on crack propagation and the crack front shape is described in many publications (Hutař et al., 2010; He et al., 2016; Oplt et al., 2019). Concerning GSSCs (sharp notches, bi-material notches, sharp material inclusions, etc.) crack initiation conditions can also be influenced by the presence of the free surface. In our study we suppose the case of a sharp notch in a homogeneous material with a straight notch front perpendicular to the free surface. In this case a crack can be initiated either from the central region of the notch front or at the vertex point (the point where the notch front intersects the free surface). In order to assess the crack initiation conditions in the 3D model, the critical quantity criterion is employed (Klusák et al., 2019). This criterion can be easily used with the knowledge of basic material parameters and with the use of results of finite element calculations of the assessed notch. It is shown that the place of crack initiation depends on the direction of external loading and thus on a mixity of normal and shear modes of loading. A comparison of results of 2D and 3D models is discussed.

High speed crack propagation characteristics of functionally graded brittle materials under ultra-high loading rate

Functionally graded materials are considered to be a special composite material. By continuously changing the composition and structure of the material, the interface and delamination problems of the material can be eliminated, which causes the material properties to change slowly with the change of the composition and structure of the material. In this paper, a lattice spring model, which can accurately show the mechanical properties of brittle materials, is used to study the evolution law and mechanism of the crack path and crack velocity in functionally graded brittle materials under ultra-high loading rate. This model uses the quantitative parameter mapping method proposed by Gusev to solve the spring stiffness coefficient of the model accurately, and the fracture criterion of the model based on the energy balance principle of Griffith is set up, using the Fortran software construct the lattice spring model for functionally graded brittle materials and carry out the numerical simulation, the evolution of crack propagation path and velocity in functionally graded brittle materials under ultra-high loading rate is studied. The simulation results show that the propagation path of high speed cracks in functionally graded brittle materials under the action of ultra-high loading rate has two stages: single crack and bifurcation crack. Once the crack reaches the critical condition of crack initiation, it will start to crack and grow at a high speed. The crack propagation velocity suddenly jumps from 0 to more than 0.5 times the Rayleigh wave velocity(CR), followed by a process of speed decline and recovery; after the initial drastic change, the growth speed gradually reaches a stable platform. Thereafter, the growth rate slowly climbed and exceeded 0.69CR; due to the stress relaxation caused by the crack initiation, the maximum tensile stress at the crack tip also showed a process of declining and slowly rising. Through comparison, it is found that the change law of the crack tip has a significant correlation with the change law of crack growth rate (high-speed growth stage), which is helpful to reveal the internal mechanism that determines the crack growth rate. The study of crack propagation under different loading rates can predict the crack propagation speed, direction and distance in functionally graded brittle materials.

Crack arrest and propagation in impact loaded shock resistant PMMA: mesh-free numerical simulation

The use of shock resistant RT-PMMA in engineering structures potentially subject to accidental overloading requires an evaluation of its crack arrest capability under impact loading. Based on experimental results obtained from a series of Kalthoff and Winkler (KW)-type impact tests, the present study aims at numerically reproducing the conditions for brittle-like crack initiation and propagation in impact-loaded RT-PMMA. For that purpose, three-dimensional SPH numerical simulations were conducted and the performance of various failure criteria was evaluated. The numerical model together with a combination of stress- and strain- dependent failure criteria were shown to fairly reproduce the experimental results in terms of finite crack advance and orientation.

Determination of fracture toughness for carbon fiber reinforced plastics free of the crack initiator using the acoustic microscopy

The fracture toughness which reflects change in the elastic deformation energy of the structural element with an increase in the crack area per unit at the onset of straining is one of the crack resistance parameters of carbon fiber plastics (CFRPs). When studying the fracture toughness, the position of the crack front is determined: both the initial one and that obtained as a result of crack growth. Currently existing test standards (STO TsAGI, ASTM D7905) determine the viscosity by the shear mode GIIc using the samples with a crack initiator. However, the method does not reflect the real conditions of crack initiation in CFRPs structures and can lead to a decrease in the accuracy of determining the load of crack initiation. A new technique of the fracture viscosity determination free of the standard delamination initiator has been developed in TsAGI. We present the results of developing the proposed methodology. The GIIc values were determined for a shear crack under three-point bending conditions after wedging. To determine the position and shape of the crack front, as well as to assess the dynamics of its propagation under subsequent loads, we used ultrasonic methods — ultrasonic flaw detection (ultrasonic NDT) and acoustic microscopy instead of the standard visual observation of the crack boundaries from the end surface of the samples. It is shown that acoustic microscopy at a frequency of 50 MHz provides determination of the crack front position in CFRP samples at a depth of 3.0 – 3.5 mm with a high resolution about 100 μm. The features of the crack growth under shear conditions are discussed. The results of the study show that high accuracy of acoustic microscopy in comparison with traditional ultrasonic NDT diagnostics is strongly sought for determining the shape of the cracks, as well as for analyzing the dynamics of crack growth and revealing the mechanisms of interlayer crack propagation in a composite material.

Instantaneous expansion with a single fracture: A new directional rock-breaking technology for roof cutting

Directional roof-cutting with shaped charge blasting is dangerous and produces large vibrations. Accordingly, a new, nonexplosive method known as instantaneous expansion with a single fracture (IESF) has been developed and tested in the field. This method uses a slit-oriented pipe and a special slitting agent triggered by electric current without a detonator. It is capable of producing a single crack that propagates through the rock in a selected direction. The mechanism of directional roof-cutting using IESF is analyzed, and the conditions for directional crack initiation and propagation are presented. The new technique has been successfully applied to directional roof-cutting in an actual coal mine. It resulted in a larger crack rate, a smaller charge, reduced vibration, and an improved safety profile than shaped charge blasting. IESFs are a substitute for explosives and are expected to have broad application prospects in directional rock breaking.

Correlation between endurance limit and cyclic yield stress determined from low cycle fatigue test

Cyclic plastic deformation is an essential condition for fatigue crack initiation. The experimental result shows that the cyclic plastic deformation takes place at the micro-level for applied stress amplitude equal to the cyclic yield stress of the metal. Physics of fatigue crack initiation justifies the fatigue crack initiation in cyclic yield stress, which is far below the tensile yield stress of the metal. Cyclic yield stress is determined from a stable stress–strain hysteresis loop of low cycle fatigue test. The author demonstrates from the experimental result of different metals collected from the literature that endurance limit and cyclic yield stress are alike.